Heavy parts in a wind turbine, such as a generator, a gearbox, or a transformer, are typically handled by terrestrial and marine cranes. Such cranes are large and bulky, so that they are not sufficiently efficient for lowering the parts from the nacelle frame of the wind turbine and hoisting them back to the nacelle frame. Such cranes are expensive so that only a small number of the cranes are available for each wind turbine having parts to be handled. As a consequence, transportation of the cranes to the site where the wind turbine is operating is usually required with the resulting added cost of transportation apart from crane renting.
Attempts for overcoming such problems have been proposed consisting in providing a crane on the nacelle frame. The crane mounted on the frame, however, is difficult to be secured thereto and to become balanced when handling a heavy part of the wind turbine, such as the generator.
Other solutions, such as that in EP1577550, have been proposed in which a winch is provided on the nacelle frame of the wind turbine. In the foregoing patent, a freely supported winch is provided on a carriage in a nacelle frame for lowering heavy parts from and raising them to the wind turbine by means of a cable. The winch used for this purpose is capital intensive since it must be quite heavy and cumbersome for withstanding the high loads in use.
International Patent Publication No. WO 02/34664 describes a solution in which a permanently mounted small, relatively inexpensive crane is used to lift a larger temporary crane that is used for heavy lifting, and International Patent Publication No. 2006/01322 describes a solution in which a self-raising crane is used. Neither of these solutions have been widely adopted because, although they solve the cost issue to some degree, they lose the flexibility of the prior art terrestrial and marine cranes. That is, they are able to effectively handle only the heavy objects that are located close to the crane position, and their position cannot easily be changed.
Thus, there remains a need for a system for handling heavy parts of wind turbines that is cost effective and, at the same time, retains the flexibility of the prior art large terrestrial and marine cranes.